Preheat the oven to 450 degrees F. Rub the lamb on all sides with the oil and season generously all over with salt and pepper. Place leg, meaty side down, on a wire rack set inside a rimmed baking pan. Roast for 15 minutes, flip it over, and roast for 15 more minutes on the other side.
Lower the oven temperature to 325 degrees F. Remove the lamb from the oven and brush the top with the mustard. Press the minced garlic, chopped rosemary, and parsley into the mustard. Continue to roast until an instant read thermometer inserted into the thickest part of the meat (but away from the bone), registers 130 to 135 degrees for medium doneness, 30 to 45 minutes (check after 30 minutes but be prepared if it takes longer!).
Transfer the lamb to a cutting board and tent it with foil. Let it rest for 15 to 20 minutes. Slice across the grain of the meat and serve.
*A semi-boneless leg of lamb has had the hipbone removed to facilitate carving.